Surge reduction filter

ABSTRACT

A surge reduction filter (SRF) includes a cartridge having a cartridge housing, a first active connection point for connection to an active line of an AC power supply, and a neutral connection point for connection to a neutral line of the AC power supply. The active and neutral connection points are located to be accessible from outside the cartridge. A first fuse and a first surge protection element are electrically connected in series between the active and neutral connection points. A status circuit is connected to monitor the surge protection element and an indicator is connected to the status circuit to indicate at least a normal status and a fault status of the surge protection element. The status circuit detects a change in voltage at a point between the fuse and the protection element and creates a fault indication if a voltage change is detected due to the fuse operating.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a national phase application under 35 U.S.C. § 371of International Application No. PCT/AU2014/000348, filed on Apr. 2,2014, which claims the benefit of Australian Provisional Application No.2013901123, filed on Apr. 2, 2013, the disclosures of which areincorporated herein by reference in their entirety.

The present invention relates to transient protection systems for powersupplies and in particular the invention provides a new configuration ofa protection device for protection against supply voltage surge and/ornoise.

BACKGROUND

Electronic equipment is generally highly reliable when operated withinenvironmental and electrical design specifications but is vulnerable tofailure when such specifications are exceeded. Electronic equipment isparticularly vulnerable to exposure to excessively high voltages, evenif exposure is extremely brief, such as when a transient spike isinjected into a power supply line. Such transients may only last formicroseconds but may expose the connected circuitry to extremely highvoltages several orders of magnitude greater than the nominal supplyvoltage. Where the load on a supply line is very large and distributed,such as is the case with distributed supplies in a residential orindustrial area, supply authorities protect supplies with air-gap surgeprotectors which remove the worst of transients from the supply network,such that any remaining transients may be absorbed by the distributedload without significant damage. However consumers are often urged toprotect sensitive equipment such as computers and expensive audio/visualequipment from residual surges with commercial power supply filters.

On the other hand, where equipment is connected to a dedicated supplyline and particularly where the equipment is at well-spaced locations ona linear supply line such as in a railway signalling system, theequipment may be exposed to significant transients due to lightningstrike. In the case of railway systems a further problem may exist wherepassing trains also inject significant noise levels into supply lines.

Railways routinely use Surge Reduction Filters (SRFs) to locally protectthe Signalling power system(s) from potentially damaging powerfluctuations as a result of supply-side lightning strikes, power surgesor other undefined transient currents. Other important infrastructurehaving sensitive equipment installations such as telephone exchanges andmobile phone transmitter towers, hospitals, computer server farms,airport control towers and military installations may also use SRFs tominimise outages due to supply fluctuations.

In the past, SRFs have incorporated protection elements (typicallymetal-oxide varistors (MOVs)) which are semi-permanently mounted usingbolted connections, thus requiring disassembly of the circuit, orswapping out of the entire SRF, when the protection element becomesunserviceable. In the case of railway signalling systems, the frequencyof replacement for the protection element in some areas may be as oftenas every 6 months. Maintenance is therefore time consuming and requiresthe supply to be powered down while maintenance is performed, which fora railway system means halting of trains. A rail network may havehundreds of SRFs distributed over a vast area.

Another issue for railway systems is that of vibration. Tracksideinstallations exist in a high vibration environment with operators oftenrequiring that equipment be rated to withstand vibration forces of atleast 11G.

High resistance connections can form due to loosening of terminalconnections due to vibration. Where bolted connections include multipleconnections on a single stud, loosening of one connection may also occurwhen another connection is tightened, inadvertently causing a highresistance contact in the loosened connection, leading to a fault.

SRFs associated with remotely located equipment may also fail withoutthe knowledge of operators, until such time as the equipment beingprotected malfunctions causing a serious safety hazard.

SUMMARY

According to a first aspect, a surge reduction filter (SRF) comprises acartridge including a cartridge housing, a first active connection pointfor connection to an active line of an AC power supply, and a neutralconnection point for connection to a neutral line of the AC powersupply, the active and neutral connection points being located to beaccessible from outside the cartridge, a first fuse and a first surgeprotection element electrically connected in series between the activeand neutral connection points, a status circuit connected to monitor thesurge protection element and an indicator connected to the statuscircuit to indicate at least a normal status and a fault status of thesurge protection element, the status circuit detecting a change involtage at a point between the fuse and the protection element andcreating a fault indication if a voltage change is detected due to thefuse operating.

In an embodiment of the SRF, the first fuse may be connected to thefirst active connection point and the first surge protection element maybe connected between the other side of the first fuse and the neutralconnection point. The cartridge may also include an earth connectionpoint, a second fuse and a second surge protection element, where theearth connection point is located to be accessible from outside thecartridge, and the second fuse and the second surge protection elementmay be connected in series between the first active connection point andthe earth connection point. The second fuse may also be connected to thefirst active connection point and the second surge protection elementmay be connected between the other side of the second fuse and the earthconnection point. The cartridge may also include third and fourth surgeprotection elements the former connected between said other side of thefirst fuse and the neutral connection point, the latter connectedbetween said other side of the second fuse and the earth connectionpoint.

In an embodiment for a three phase power supply, the cartridge mayinclude a second active connection point for connection to a differentphase of the AC power supply with respect to the phase of the firstactive connection point with the second active connection point alsobeing located to be accessible from outside the cartridge. In this casethe second fuse and the second surge protection element, may beconnected in series between the first active connection point and thesecond active connection point with one side of the second fuseconnected to the first active connection point and the second surgeprotection element connected between the other side of the second fuseand the second active connection point. Similarly, a third surgeprotection element may be connected between said other side of the firstfuse and the second active connection point and a fourth surgeprotection element may be connected between said other side of thesecond fuse and the neutral connection point.

The status circuit may monitor the state of each of the first and secondfuses and produce a different fault indication when one fuse is operatedcompared to a fault indication produced when two fuses are operated.

The cartridge for both single phase and three phase systems may alsoinclude a fifth surge protection element connected between the neutralconnection point and the earth connection point. Optionally thecartridge may also include a sixth surge protection element connectedbetween the neutral connection point and the earth connection point butthis second (i.e. redundant) neutral to earth protection element may beomitted from cartridges for 3 phase systems as there will still be 3neutral to earth protection elements provided by 3 cartridges.

The SRF may include a cartridge tray into which the cartridge isinsertable. The cartridge tray may have connectors projecting from itsbase, which co-operate with connectors projecting from the cartridge andwhich are associated with the contact points of the cartridge, wherebythe connectors in the tray and the cartridge connect the circuit in thecartridge to an external circuit comprising the AC supply. The cartridgetray and the cartridge may be co-operatively keyed for correctorientation of insertion of the cartridge into the cartridge tray andthe connector location in the cartridge tray and cartridge may be variedaccording to a cartridge voltage rating to co-operatively key thecartridge and cartridge tray to prevent connection of an incorrectlyrated cartridge.

Each cartridge may include a status indicating LED, which indicates astatus of the surge protection elements monitored by the status circuit,by changing a state of emission of the LED. The cartridge may alsoinclude a remote monitoring output connected via a monitoring connectionpoint with co-operating connectors projecting from the base of thecartridge tray and the cartridge to connect the remote monitoring outputto a remote monitoring interface.

According to a second aspect, a surge reduction filter comprises a firstraw side active connector of one AC power phase, a first clean sideactive connector, a raw side neutral connector, a clean side neutralconnector, a first single solid active conductor connecting the firstraw side active connector to the first clean side active connector, thefirst single solid active conductor forming a winding of an inductorintermediate its ends, a single solid neutral conductor connecting theraw side neutral connector to the clean side neutral connector, thesingle solid neutral conductor forming a winding of an inductorintermediate its ends, a first surge protection element connectedbetween the first clean side active connector and the clean side neutralconnector.

The SRF may include a capacitor connected between the first clean sideactive connector and the clean side neutral connector. A printed circuitboard is provided having tabs inserted into and connecting with therespective first clean side active connector, the clean side neutralconnector and an earth connector and the capacitor may be mounted on theprinted circuit board between the first clean side active connector andthe clean side neutral connector.

Embodiments of the SRF use ‘screwless type connectors with directpressure’ (as defined in Annex D of IEC 60947_1:2004. “Low-voltageswitchgear and control gear Part 1: general Rules” and referred toherein as “screwless type connectors”) for the first clean side activeconnector, the clean side neutral connector and the earth connectorallowing the tabs of the printed circuit board to plug into voltage tapinputs of the screwless type connectors.

The surge protection element may be mounted in a cartridge plugablyconnectable to the printed circuit board. The cartridge may include acartridge housing, a first active connection point, and a neutralconnection point, the first active connection point and the neutralconnection point being located to be accessible from outside thecartridge and plugably connectable to the clean side of the activeconductor and the clean side of the neutral conductor respectively viathe printed circuit hoard. A first fuse and the first surge protectionelement may be electrically connected in series between the active andneutral connection points. A status circuit may be connected to monitorthe surge protection element and an indicator connected to the statuscircuit may indicate at least a normal status and a fault status of thesurge protection element when the status circuit detects a change involtage at a point between the fuse and the protection element to createa fault indication if a voltage change is detected due to the fuseoperating.

The first fuse may be connected to the first active connection point andthe first surge protection element may be connected between the otherside of the first fuse and the neutral connection point. The cartridgemay also include an earth connection point, a second fuse and a secondsurge protection element, with the earth connection point being locatedto be accessible from outside the cartridge and plugably connectable tothe earth connector via the printed circuit board. The second fuse andthe second surge protection element may be connected in series betweenthe first active connection point and the earth connection point. Thesecond fuse may also be connected to the first active connection pointand the second surge protection element may be connected between theother side of the second fuse and the earth connection point.

The cartridge may also include a third surge protection elementconnected between said other side of the first fuse and the earthconnection point and a fourth surge protection element connected betweensaid other side of the second fuse and the neutral connection point.Fifth and optionally sixth surge protection elements may be connectedbetween the neutral connection point and the earth connection point.

In a three phase embodiment the surge reduction filter may furthercomprise a second raw side active connector, a second clean side activeconnector, a third raw side active connector, a third clean side activeconnector, a second single solid active conductor connecting the secondraw side active connector to the second clean side active connector, athird single solid active conductor connecting the third raw side activeconnector to the third clean side active connector, the second and thirdsingle solid active conductors each forming a winding of an inductorintermediate their ends, with surge protection elements connectedbetween the second clean side active connector and the clean sideneutral connector and between the third clean side active connector andthe clean side neutral connector.

Capacitors may be connected between each clean side active connector andthe clean side neutral connector. A printed circuit hoard is providedhaving tabs inserted into and connecting with the respective firstsecond and third clean side active connectors, the clean side neutralconnector and an earth connector and the capacitors may be mounted onthe printed circuit hoard between the first, second and third clean sideactive connectors and the clean side neutral connector. Embodiments ofthe SRF use ‘screwless type connectors with direct pressure’ for thefirst second and third clean side active connectors, the clean sideneutral connector and the earth connector allowing the tabs of theprinted circuit board to plug into voltage tap inputs of the screwlesstype connectors.

The surge protection elements may be mounted in three identicalcartridges plugably connectable to the printed circuit board, eachcartridge providing surge protection between one phase active andneutral and between said one phase active and another phase active, eachof the three cartridges being connectable to one of three phases andbetween one pair of three phase pairs, with the printed circuit boardconnecting different phases and phase pairs to each cartridge, wherebyin combination the three cartridges provide phase to neutral protectionfor each of the phases and phase to phase protection between each phasepair of a three phase AC power supply.

Each of the three cartridges may include a cartridge housing, a firstactive connection point, and a neutral connection point, the firstactive connection point and the neutral connection point being locatedto be accessible from outside the cartridge and plugably connectable tothe clean side of the active conductor and the clean side of the neutralconductor respectively via the printed circuit board. A first fuse andthe first surge protection element may be electrically connected inseries between the active and neutral connection points. A statuscircuit may be connected to monitor the surge protection element and anindicator connected to the status circuit may indicate at least a normalstatus and a fault status of the surge protection element, by detectinga change in voltage at a point between the fuse and the protectionelement and creating a fault indication if a voltage change is detecteddue to the fuse operating.

The first fuse may be connected to the first active connection point andthe first surge protection element may be connected between the otherside of the first fuse and the neutral connection point. Each cartridgemay also include a second active connection point for connection to adifferent phase of the three phase AC power supply with respect to thephase of the first active connection point, a second fuse and a secondsurge protection element. The second active connection point may belocated to be accessible from outside the cartridge and plugablyconnectable to the clean side of the second active conductor via theprinted circuit board, and the second fuse and the second surgeprotection element may be connected in series between the first activeconnection point and the second active connection point. One side of thesecond fuse may also be connected to the first active connection pointand the second surge protection element may be connected between theother side of the second fuse and the second active connection point.

The cartridge may include a third surge protection element connectedbetween said other side of the first fuse and the second activeconnection point and may also include a fourth surge protection elementconnected between said other side of the second fuse and the neutralconnection point.

The status circuit monitors the state of each of the first and secondfuses and produces a different fault indication when one fuse isoperated compared to a fault indication produced when two fuses areoperated.

The cartridge may also include an earth connection point, located to beaccessible from outside the cartridge and plugably connectable to anearth connector via the printed circuit board. A fifth surge protectionelement may be connected between the neutral connection point and theearth connection point.

The SRF may include three cartridge trays into which the threecartridges are insertable, with connectors projecting from a base ofeach cartridge tray co-operating with connectors projecting from eachcartridge to connect the circuits in the cartridges to an externalcircuit comprising the AC supply. The cartridge trays and the cartridgesmay be co-operatively keyed for correct orientation of insertion of thecartridges into the cartridge trays. The connector locations in thecartridge trays and cartridges may also be varied according to acartridge voltage rating to co-operatively key the cartridges andcartridge trays to prevent connection of incorrectly rated cartridges.

The cartridge, or each cartridge, may include a status indicating LED,which indicates a status of the surge protection elements monitored bythe status circuit, by changing a state of emission of the LED. Thecartridge, or each cartridge, may also include a monitoring connectionpoint and the monitoring circuit may include a remote monitoring outputconnected to the monitoring connection point, with co-operatingconnectors projecting from the base of the cartridge tray and thecartridge associated connecting the remote monitoring output to a remotemonitoring interface via the printed circuit board.

For each of the permutations of devices disclosed above, a furthervariation may be provided in which at least one fuse is connected to araw active supply. In the case of the fuse connected to the raw activesupply, protection elements may be connected from the fuse to a rawneutral line and/or a RAW adjacent phase line and/or an earth line ofthe AC supply. Monitoring of the status of the fuses is also similarlyprovided, however in the case of the fuse connected to the raw activesupply, opto-coupling may be provided between the point monitored andthe monitoring circuit for isolation.

The fuse or fuses connected to the raw active supply may be in additionto, or instead of, one or more fuses connected to the clean activesupply as discussed above. In the case of the fuse connected to theclean active supply, protection elements may be connected from the fuseto a clean neutral line and/or a clean adjacent phase line and/or anearth line of the AC supply.

Protective devices may also be connected between a clean and/or rawneutral and earth.

Each of the surge protection elements may be an MOV.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A & B provide an electrical schematic drawing of an example of asingle phase SRF device in which;

FIG. 1A show a chassis circuit, and

FIG. 1B shows the motherboard and MOV cartridge circuits;

FIG. 2 is a perspective view of a back plate (chassis) with busbars andinductors for a three-phase SRF device of similar design to the singlephase device shown in FIG. 1;

FIG. 3 is a perspective view of the 3-phase hack plate (chassis) of FIG.2 with a mother board fitted;

FIG. 4 is a perspective view of the 3-phase back plate (chassis) of FIG.2 with three cartridge trays fitted and MOV cartridges fitted to two ofthe cartridge trays;

FIG. 5 is an exploded view of a MOV cartridge suitable for the chassisexample shown in FIG. 4;

FIG. 6 is a circuit schematic of an example of a MOV cartridge for athree phase system.

FIGS. 7A & B provide an electrical schematic drawing of an alternativeexample of a single phase SRF device in which;

FIG. 7A shows a chassis and auxiliary circuit board circuits, and

FIG. 7B shows the motherboard and MOV cartridge circuits;

FIG. 8 is a perspective view of the 3-phase back plate (chassis) of FIG.2 with a mother board and auxiliary board fitted in accordance with analternative protection arrangement which is illustrated for the singlephase case (for simplicity) in the schematic of FIGS. 7A and 7B;

FIG. 9 is a perspective view of the 3-phase back plate (chassis) of FIG.8 with three cartridge trays fitted and MOV cartridges fitted to two ofthe cartridge trays;

FIG. 10 is an exploded view of a MOV cartridge suitable for the chassisexample shown in FIG. 9;

FIG. 11 is a circuit schematic of a 3-phase MOV cartridge which would beused in a 3-phase equivalent of the alternative single phase systemshown in FIGS. 7A & 7B

DETAILED DESCRIPTION OF AN EMBODIMENT Example 1

A circuit schematic for a first example of a single phase SRF isillustrated in FIG. 1. A three phase circuit will replicate thecomponents of the single phase circuit three times with the exceptionthat the three phase circuit has phase to phase protection and does nothave phase to earth protection and neutral and earth connections andcircuits are common to the three phases in the three phase example. Thecomponent values may be changed to achieve different voltage and currentratings but the circuit configuration will remain similar.

Customer power wiring is connected to the three-phase SRF by means of 9‘screwless type connectors with direct pressure’ (as defined in AppendixD of IEC 60947_1:2004. “Low-voltage switchgear and control gear Part 1:general Rules”), (noting that there are only 5 connections for a singlephase system). Screwless type connectors, which accommodate wires up to50 mm², are typically used for the clean or filtered side (down stream)connectors. Long upstream feeder line voltage drop requirements in somecases dictate the use of heavier-current conductors and thereforeheavier-current connectors for the raw side connections to the SRF thanare required for the clean (down stream) side. Screwless type connectorswhich accommodate wires up to 95 mm², may be used on the raw side.

Power is input to a raw side neutral connector 101 and a raw side active(or line) connector 102. The raw side connectors 101 and 102 are alsoseen in FIG. 2, which depicts a bus bar assembly of a 3-phase SRF. InFIG. 2 the Active connector 102 is provided for first phase andconnectors 103 & 104 are active connectors for second and third phases.The active connectors 102, 103, 104 and the neutral connector 101 aremounted on a standard DIN rail 231, which is in turn mounted on a metalback-plate 232. An earth connector 125 is also located on the DIN rail231. In the three phase case the circuits for each of the three phasesare similar in topology to that of the single phase (with the exceptionsreferred to above) and will be described simultaneously below (byinclusion of additional reference numerals where appropriate).

A raw side end 105 of a neutral busbar is connected to the raw sideneutral connector 101 and a raw side end 106 (107, 108) of an activebusbar is connected to the raw side active connector 102 (103, 104). Theraw side end 105 of the neutral busbar is in fact a long tail (input) ofa wound (10-30 μH) inductor 109 (L2/1) and the second tail (output) 113of the inductor 109 forms the clean side end of the neutral busbar andis connected to a clean side neutral connector 121 such that theconnection from the raw side neutral connector 101 to the clean sideneutral connector 121 is a single piece of solid metal conductor with nointermediate joins or connections. Similarly The raw side end 106 (107,108) of the active busbar is in fact a long tail (input) of a wound(10-30 μH) inductor (L1/1) 110 (L1/2, L1/3—111, 112) and the second tail(output) 114 (115, 116) of the inductor 110 (111, 112) forms the cleanside end of the active busbar and is connected to a clean side activeconnector 122 (123, 124) such that the connection from the raw sideactive connector 102 (103, 104) to the clean side active connector 122(123, 124) is a single piece of solid metal conductor with nointermediate joins or connections. The clean side connectors are alsopreferably screwless type connectors capable of accommodating up to 50mm² conductors. The raw side connectors are also preferably screwlesstype connectors and will be specified by the customer depending upon theup stream supply line length (e.g. connectors capable of accommodatingup to 95 mm² conductors may be specified if the upstream supply linelength is long). By providing a single unbroken conductor between theinput (raw side) connectors and the output (clean side) connectors thenumber of connections is minimised thereby reducing the potential forconnector related faults in the power path. Also by using solidconductors in the primary current path, problems associated withconnecting stranded conductors are avoided. The inductors 109, 110, 111& 112 are mounted to the metal back-plate 232, as are the insulationblocks 233 which locate the busbar ends 105, 106, 107, 108, 113, 114,115 & 116.

The remainder of the circuit of FIG. 1 comprises filter capacitors, MOVsand a status detection and status reporting circuit. The filtercapacitors are mounted on the Mother Board (301 of FIG. 3) and theMother Board is connected directly into the clean side connectors (cleanside neutral connector 121, the clean side active connector 122 (123,124)) and the earth connector 125. The MOVs and a status detectioncircuit are mounted within a Varistor Cartridge 511 (512, 513—See FIGS.4 & 5) on a Varistor Cartridge PCB (608 of FIG. 5) and the VaristorCartridge 511 (512, 513) is connected to the Mother Board 301. Areporting circuit (Remote Monitor Interface) is mounted in a plastichousing 241 located on the DIN rail 231 and connected to the statusdetection circuit on the Varistor Cartridge PCB 608 via the Mother Board301 and a cable 242.

Referring to FIG. 3, connection of the Mother Board PCB 301 to theconnectors 121, 122, 123, 124 & 125 is via voltage tap input point (351,352, 353, 354, 355) of each connector using appropriately sized tabs onthe Mother Board 301 (i.e. a neutral tab 341, an active tab 342 (343,344) and an earth tab 345. Double-sided and mirrored 6 oz copper is usedon the Mother Board 301, connecting the connection tabs (341, 342, 343,344, 345) to the other components, to enable the copper traces to carrythe expected surge currents. The screwless type connectors (101, 102,103, 104, 121, 122, 123, 124, 125) used in this embodiment have a tappoint above the main connection opening, which are secondary connectionpoints that permit a flat auxiliary conductor to be inserted into theconnection point above the main connection point and provided under thesame tensioning mechanism as the main connection such that it is clampedwith the same clamping force as the main connection to provide reliableconnection between the internal conductor of the connector (connectingthrough to the other side of the connector), the bus bar inserted intothe main connector opening and the auxiliary conductor (which is in thiscase a tab (341, 342, 343, 344, 345) of the Mother Board 301). Thevoltage tap inputs of the screwless type connectors are not intended tocarry the full load current passing through the connector, and aretypically used to connect monitoring equipment and the like. In thiscase the voltage tap inputs must carry intermittent surge currents whichmight be many times greater than the normal load current suppliedthrough the SRF but will only last for and extremely short period oftime.

Capacitors C2 and C3 (C2/1, C3/1, C2/2, C3/2, C2/3 & C3/3 in FIGS. 3 & 4for the 3-phase embodiment), which may be typically in the range of10-50 μf, are mounted on the Mother Board and connected between theclean active busbar 114 (115, 116) and the clean neutral busbar 113 tofurther condition the power after the current passes through theinductors 109 and 110 (111, 112). Bleed resistors (not shown) areprovided to discharge the capacitors C2 and C3 (C2/1, C3/1, C2/2, C3/2,C2/3 & C3/3) when power is removed after production testing and serve nopurpose in use (unless disconnection of the unit is required at sometime).

Referring to FIG. 4, one Varistor Cartridge Tray 501 (502, 503) ismounted on the Mother Board 301 for each phase of the supply. A VaristorCartridge 511 (one removed in FIG. 5) (512, 513) is carried in eachVaristor Cartridge tray 501 (502, 503) and contains the surge protectionelements (MOVs) and status monitoring and indicating circuits, whichwill be described below. The Varistor Cartridge 511 (512, 513) allowshot removal and replacement of the MOVs protecting the load, withoutdisruption of the supply to the load. In the case of a railwaysignalling system this permits the railway to continue operation whilereplacement is being performed without danger to rail crews, passengersor maintenance staff. Connection to the circuitry within each VaristorCartridge 511 (512, 513) is via covered co-operating banana sockets 521,522, 523, 524 & 525 and plugs 511, 512, 513, 514 & 515 mountedrespectively on the Mother Board 301 and an internal PCB 608 of theVaristor Cartridge. The following connection points are provided foreach Varistor Cartridge of a 3 phase system:

1) Clean Neutral 404 (405, 406);

2) Earth 401 (402, 403);

3) Clean Active (adjacent phase), (not shown in FIG. 1 which only showsa single phase system);

4) Remote Monitor Signal Output 421 (422, 423);

5) Clean Active 407 (408, 409).

Each of these connection points on the Mother Board are fitted with asocket for a banana plug with the mating plug connected to the circuitin the mating cartridge. Referring to FIG. 4 the banana sockets areillustrated for one phase as follows:

1) Clean Neutral 521;

2) Earth 522;

3) Clean Active (adjacent phase) 523;

4) Remote Monitor Signal Output 524;

5) Clean Active 525.

Location of these connectors on the Mother Board 301 will vary slightlydepending on the specification and voltage rating of the SRF to providekeying to prevent connection of an incorrect cartridge.

A locking-type 4-pin connector socket 244 is soldered onto a landing 424on the Mother Board 301 (seen in FIGS. 3 & 4 and also referenced inFIGS. 1A & 1B) to provide connection of the Remote Monitor Interface. Aswell as the 3 remote status monitoring signals, a clean neutral is takenout through the 4-pin connector 244 to provide a signal return path.Referring to FIGS. 2, 3 and 4, a 4-pin plug 243 on the free end of a4-core cable 242 connects the Remote Monitor Interface in housing 241 tothe Mother Board 301 via the 4-pin socket 244. The housing 241 mountedon the DIN rail 231 houses the Remote Monitor Interface electronicsincluding the Solid State Relays SSR1 & SSR2 and a triple screwless typeconnector providing voltage free contacts which can be wired to providestatus signals to a remote monitoring system (see FIG. 1).

Referring to FIG. 5, an exploded view of the Varistor Cartridge 511(512, 513) for a 3 phase system is illustrated. Three such cartridgesare used in a 3 phase system and the cartridge for a 3 phase system isphysically similar to a cartridge for a single phase system but has someminor variations related to keying of the cartridge with the tray 501(502, 503) and the circuit within the cartridge is modified to providephase to phase surge reduction. The Varistor Cartridge comprises ahousing base 601 and housing cover 602 which includes a handle portion603. The base 601 and cover 602 are joined by resilient clips 605 andscrews 607. The screws 607 engage through the PCB 608 within thecartridge, with threaded inserts 609 which are themselves screwed intothe cover 602, to secure the base 601 to the cover 602 and locate thePCB between the base 601 and the cover 602. Additional screws 606 holdthe PCB to the cover 602 when the base 601 is removed. MOVs and othercircuit components are mounted on the Varistor Cartridge PCB 608 asshown in FIG. 6 (the equivalent componentry in FIG. 1 is similarlymounted in a cartridge for a single phase system). LED1 is mounted on anedge of the Varistor Cartridge PCB 608 and a transparent rod 621 islocated above LED1 to act as a light pipe. The upper end of the rod 621projects into the window 622 in the cover 602 to provide a visualindication of the status of the MOVs in the cartridge (using amonitoring scheme as described below)

Banana plugs extend below the Varistor Cartridge PCB 608 to connect withsockets 521, 522, 523, 524 & 525 connected to the Mother Board 301 andwhich extend through the cartridge tray 501 (502, 503). The circuitsconnected via these plugs and sockets are set out below:

Socket Plug Connection (tray) (Cartridge) 1 Clean Neutral 521 611 2Earth 522 612 3 Clean Active (adjacent phase) 523 613 4 Remote MonitorSignal Output 524 614 5 Clean Active 525 615

The earth plug 612 is physically longer than the other plugs of thecartridge such that this connection makes first and breaks last.

Varistor Cartridges may be for single phase or 3 phase systems and maybe rated for 415V, 240V or 120V inputs. The locations of the 415, 240Vand 120V inputs on the Mother Board discriminate between 3 phase orsingle phase and 415, 240 or 120V Varistor Cartridges so that only acorrectly specified Varistor Cartridge can be inserted into a particularSRF.

MOV and Status Circuit

The operation of a Varistor Cartridge will now be described withreference to FIGS. 1 and 6. FIG. 1 is a composite electrical schematicdrawing for a single phase unit but has been marked with references forequivalent points in the second and third phase circuits of a 3 phasesystem. However it will be noted with reference to FIG. 6 that althoughin a three phase system, there are three cartridges, there are somedifferences in the Varistor Cartridge of a 3 phase system. In particularthe cartridge of a 3 phase system includes a phase to phase MOV (Phase Nto Phase N−1 in each cartridge). Also in the cartridge of the 3 Phasesystem, there may be no phase-to-earth MOVs (MOV5 & MOV6) and there mayonly be only one earth to neutral MOV (MOV3—as each of the 3 cartridgeshave one of these there are in fact 3 per system).

1) The filtered (clean) active supply 114 is connected to the MOV stackthrough fuses FS1 and FS2. (The MOV's are arranged so that in each pairconnected to one fuse there is a MOV between Active and Neutral andanother between Active and Earth). Fuse FS1 monitors MOV 1 and MOV 5 forshort circuit while fuse FS2 monitors MOV2 and MOV6.2) In the 3 phase circuit of FIG. 6, MOV5 & MOV6 are absent and MOV7 andMOV8 provide phase to phase protection between the assigned phase of thecartridge and the adjacent phase. MOV1 and MOV7 in this case aremonitored by fuse FS1 and MOV2 and MOV8 are monitored by fuse FS2.3) In the single phase circuit (FIG. 1) two MOVs (MOV3 and MOV4) areconnected between neutral and earth. In the 3 phase case (FIG. 6) MOV4is omitted as the three SRF cartridges each carry a neutral-to-earthMOV.4) As described in paragraphs 1)-3) above, there are two MOVs betweenActive and Neutral, Active and Earth (or phase to phase) and one or twoMOVs between Neutral to Earth. This arrangement provides redundancy suchthat in the event that one MOV in any pair becomes unserviceable (andthe corresponding fuse operates) a second MOV remains to protect thesupply until the Varistor Cartridge can be replaced.5) The circuit comprising capacitor C1, zener diode Z1, diode D1,resistors R1, R2, R3 and capacitor C4 is a voltage limited half-waverectifier and filter that supplies dc power to the B-series CMOS SchmittTrigger input logic element packages U1 and U2 (“N+11V”).6) Capacitor C1 acts as a capacitive voltage dropper that limits themaximum current through Z1 to within its rating. For the purposes ofprotecting Testing Staff from high-voltage shocks, R2 and R3 dischargeC1 after production testing, they serve no function during normaloperation.7) Resistor R1 limits the maximum forward surge current through diode D1to a value within the rating of the diode.8) Diode D1 itself prevents capacitor C4 from being discharged onnegative half-cycles of the filtered mains during which the zener diodeZ1 becomes forward-biased, such that its cathode is pulled down to about−700 mV with respect to Filtered Neutral 113.9) Two identical fuse monitoring circuits are provided, one for eachfuse. The circuit that monitors fuse FS1 comprises capacitor C7, zenerdiode Z2, diode D3, resistors R5, R8, R9, R10 and capacitor C6. Thecircuit that monitors fuse FS2 is made-up of C8, zener diode Z3, diodeD6, resistors R17, R18, R19, R22 and capacitor C10. The topology of boththese circuits is identical to that of the “N+11V” power supply circuitalready discussed; the only differences are in the values of thedropping capacitors C7 and C8, and the bleeder resistors shunting theoutput capacitors C6 and C10. The circuits independently produce thelogic signals FS1OK and FS2OK that in the active HI state indicate thatfuses FS1 and FS2 respectively are intact.10) Inverter U1C, diode D2, capacitor C5 (1 μF) and resistor R4 form anon/off-controlled oscillator having a frequency of about 1 Hz with aduty cycle of close to 50%. Inverter U1D, diode D4, capacitor C9 andresistor R16 form another such oscillator running at about 5 Hz with thesame sort of duty cycle.11) Under normal circumstances when the Varistor Cartridge 511 (512,513) is fully populated for dual MOV redundancy, a link LK1 will be leftopen circuit, and with both fuses intact signals FS1OK and FS2OK areboth HI. So the output of NAND gate U2A is LO thus driving InverterU1C's input LO through diode D2 to disable the 1 Hz oscillator, anddrive the output of Inverter U1C HI.12) Inverters U1A and U1B and NAND gate U2C form a simple equivalent ORgate whose output is inverted by inverter-wired NAND gate U2D. Becausesignals FS1OK and FS2OK are both normally HI, the output of gate U2Cwill also be HI and the output of gate U2D will be LO. This LO signalout of gate U2D will pull the input of inverter U1D LO through resistorR14 and diode D4 thereby disabling the 5 Hz oscillator. The HI at theoutput of Inverter U1D reverse-biases diode D5 so that it effectivelydisconnects the output of Inverter U1D from the junction of resistorsR12 and R15. (Because R14 is much less than R16, the LO at the output ofNAND gate U2D will pull the input of Inverter U1D well down below itspositive-going threshold).13) The HI at the output of Inverter U1C thus energises LED1 throughresistor R12 and R15 and LED1 glows steadily.14) The HI levels at the outputs of Inverter U1C and NAND gate U2C drivethe output of NAND gate U2B LO. This is inverted to HI by Inverters U1Eand U1F to become the logic signal RMIN. Inverters U1E and U1F areshunted together in order to double the drive capability of RMIN.15) Therefore in the normal condition with both fuses FS1 and FS2intact, LED1 glows steadily and RMIN is HI.16) In the event of the Cartridge intercepting a transient that overstresses one of the MOV's to the extent that it becomes short circuit,the fuse monitoring that particular MOV will operate and the now faultyMOV will be isolated.17) As an example, assume that MOV 1 has failed to a short circuit.18) Fuse FS1 operates and becomes open circuit. After a few seconds,signal FS1OK falls below the negative-going threshold of NAND gate U2Apin 1 and the output of NAND gate U2A will be driven HI.19) Diode D2 is thus reverse-biased thereby removing the LO level at theinput of Inverter U1C and allowing capacitor C5 to charge throughresistor R4.20) When the voltage at the input of Inverter U1C rises above itspositive-going threshold, its output is forced LO. Capacitor C5 nowdischarges through resistor R4 until its voltage again falls below thenegative-going threshold of Inverter U1C, whereupon its output is drivenHI and the whole cycle starts over again.21) (Meanwhile, there has been no change of logic state at the output ofNAND gate U2C because signal FS2OK is still HI meaning that the 5 Hzoscillator is still disabled.)22) The 1 Hz rectangular wave at the output of Inverter U1C causes LED1to flash at that rate indicating that a MOV inside that Cartridge hasfailed, the level of protection has been degraded and the Cartridgeshould be replaced. (However, the second MOV in the affected pair (MOV2in this example) is still in service and the load is still protected).23) Signal RMIN will also oscillate at 1 Hz and drive the Remote MonitorInterface accordingly.24) Should another equally destructive transient strike and cause fuseFS2 to also rupture, signal FS2OK soon falls to a LO level and theoutput of NAND gate U2C will be driven LO. The output of NAND gate U2Dresponds by going HI, reverse-biasing diode D4 and allowing the 5 Hzoscillator to run. Now both oscillators are running.25) When the rectangular wave now at the output of Inverter U1D is LO,diode D5 is forward-biased and the junction of resistors R12 and R15 ispulled-down to such a level as would prevent LED1 from glowing even ifInverter U1C was trying to turn it on. So the situation were both fuseshave operated is indicated by LED1 flashing 5 times in the first (HI)half of a 1 Hz oscillator cycle, and staying dark for the second half.This kind of flashing activity is from now on called the “burst”pattern.26) The LO that now appears at the output of NAND gate U2C drives theoutput of NAND gate U2B HI and therefore signal RMIN is driven LO.Remote Monitor Interface27) The (optional) Remote Monitor Interface assembly is designed to bedeployed in either single- or 3-phase SRF's. Its specific purpose is toprovide a means of reporting to some off-site location the condition ofthe Varistor Cartridge and hence the level of protection of the loadconnected to the SRF.28) In the single-phase case as depicted, the 3 independent inputs tothe Remote Monitor Interface are shorted together. So when signal RMINis LO, NPN bipolar junction transistors Q1 and Q2 are both cut-off andthe voltage at the junction of resistors R23 and R24 is zero, hencethere is no path for current through LED2 or through the input LED's ofsolid state relays SSR1 and SSR2, whose respective output contacts arein their de-energised states.29) When signal RMIN is HI, NPN transistors Q1 and Q2 are bothsaturated, and the junction of resistors R23 and R24 is connected toN+11V. Current can now flow through LED2 and the input LEDs of the solidstate relays SSR1 and SSR2 so LED2 glows and the output contacts ofsolid state relays SSR1 and SSR2 are driven into their respectiveenergised states.30) The truth table presented below summarises the behaviour of LED1 andsignal RMIN in response to all combinations of fuse states for a fullyequipped dual redundancy Varistor Cartridge.

FS1 (FS1OK) FS2 (FS2OK) LED1 PATTERN RMIN/LED2 OPERATED OPERATED BURSTOFF OPERATED INTACT 1 Hz FLASH ON/OFF at 1 Hz INTACT OPERATED 1 Hz FLASHON/OFF at 1 Hz INTACT INTACT ON STEADY ON STEADYHalf-Equipped Varistor Cartridge31) Provision is made for Varistor Cartridges to be fitted with singleMOV's between Active and Neutral, Active and Earth and Neutral and Earthby omitting MOV2, MOV4 and MOV5 (or Active to Active by omitting MOV8).(Although both fuses are installed, only that which monitors the singleVaristor Stack (FS1) is ever likely to blow, FS2 just drives FS2OK HI).32) In this case the shorting link LK1 must be fitted causing thebehaviour of LED1 to differ slightly in that it can now only display theBurst pattern when FS1 has operated.33) Link LK1 shorts the cathode of diode D4 to the output of NAND gateU2A. This combined with resistor R14 being in series with the output ofNAND gate U2D hands control of the 5 Hz oscillator to NAND gate U2A.34) If both fuses are intact then LED1 glows steadily and signal RMIN isa constant HI, as for the fully equipped case.35) If either fuse operates, the output of NAND gate U2A will become HIand diodes D2 and D4 will both be reverse-biased and both oscillatorswill run, thus resulting in LED1 displaying the Burst pattern, thebehaviour of signal RMIN is unaffected by LK1 being installed (see thetruth table below).

FS1 (FS1OK) FS2 (FS2OK) LED1 PATTERN RMIN/LED2 OPERATED OPERATED BURSTOFF OPERATED INTACT BURST FLASH INTACT OPERATED BURST FLASH INTACTINTACT ON STEADY ON STEADY36) In some situations, like where an un-snubbed inductive device isconnected to the RMI's volt-free contacts (a Q-style relay forinstance), high levels of back emf induced by continual ON/OFF periodicswitching may upset nearby electronic systems.37) Installing link LK2 disables the flashing of the signal RMIN infavour of the OFF pattern as follows.38) Link LK2 connects the cathode of diode D7 to the inputs of InvertersU1E and U1F.39) When both fuses FS1 and FS2 are intact, the output of NAND gate U2Ais LO so diode D7 is reverse-biased.40) If either fuse FS1 or FS2 operates, the resulting HI on the outputof NAND gate U2A forward-biases diode D7 which now connects that HI tothe inputs of U1E and U1F and signal RMIN is driven LO (OFF).41) A summary of the behaviour of RMIN/LED2 and LED1 in response to thestates of LK1 and LK2 is tabulated below. In this arrangement, Link LK1determines the level of protection and Link LK2 determines whether RMINis allowed to oscillate and LED1 is allowed to flash.

RMIN, PROTEC- RMIN LED2 TION LK1 LK2 FUSES LED2 LED1 FLASHING LEVEL OUTOUT INTACT ON ON ENABLED DUAL ONE FLASH FLASH VARIS- OPERATED TOR BOTHOFF BURST STACK OPERATED REDUN- IN INTACT ON ON DISABLED DANCY ONE OFFFLASH OPERATED BOTH OFF BURST OPERATED IN OUT INTACT ON ON ENABLEDSINGLE ONE FLASH BURST VARIS- OPERATED TOR BOTH OFF BURST STACK OPERATEDIN INTACT ON ON DISABLED ONE OFF BURST OPERATED BOTH OFF BURST OPERATED

Example 2

An alternative circuit schematic for a second example of a single phaseSRF is illustrated in FIG. 7. As with the FIG. 1 example, a three phasecircuit will replicate the components of the single phase circuit threetimes with the exception that the three phase circuit has phase to phaseprotection and does not have phase to earth protection and neutral andearth connections and circuits are common to the three phases in thethree phase example. As with the previous example, component values maybe changed to achieve different voltage and current ratings but thecircuit configuration will remain similar.

The same chassis arrangement of FIG. 2 is used in this second examplebut the mother board and an auxiliary circuit board attached to thechassis (see FIG. 8) are different to achieve the altered circuitarrangement of FIGS. 7A & 7B (or its 3-phase equivalent). As seen inFIG. 2 customer power wiring is connected to the three-phase SRF bymeans of 9 ‘screwless type connectors with direct pressure’ (as definedin Appendix D of IEC 60947_1:2004. “Low-voltage switchgear and controlgear Part 1: general Rules”), (noting that there are only 5 connectionsfor a single phase system). Screwless type connectors, which accommodatewires up to 50 mm², are typically used for the clean or filtered side(down stream) connectors. Long upstream feeder line voltage droprequirements in some cases dictate the use of heavier-current conductorsand therefore heavier-current connectors for the raw side connections tothe SRF than are required for the clean (down stream) side. Screwlesstype connectors which accommodate wires up to 95 mm², may be used on theraw side.

The description of FIG. 2 above will not be repeated here as thecomponents of FIG. 2 are essentially the same for both examples.

The remainder of the circuit of FIG. 7 comprises filter capacitors, MOVsand a status detection and status reporting circuit. The filtercapacitors are mounted on the Mother Board (801 of FIG. 8) and theMother Board is connected directly into the clean side connectors (cleanside neutral connector 121, the clean side active connector 122 (123,124) and the earth connector 125). Additionally an auxiliary circuithoard 802 is connected to the raw side connectors (raw side neutralconnector 101, the raw side active connector 102 (103, 104)). The MOVsand a status detection circuit are mounted within a Varistor Cartridge911 (912, 913—See FIGS. 9 & 10) on a Varistor Cartridge PCB (1008 ofFIG. 10) and the Varistor Cartridge 911 (912, 913) is connected to theMother Board 801. A reporting circuit (Remote Monitor Interface) ismounted in a plastic housing 241 located on the DIN rail 231 andconnected to the status detection circuit on the Varistor Cartridge PCB1008 via the Mother Board 801 and a cable 242.

Referring to FIG. 8, connection of the Mother Board PCB 801 to theconnectors 121, 122, 123, 124 & 125 is via voltage tap input point (351,352, 353, 354, 355) of each connector using appropriately sized tabs onthe Mother Board 301 (i.e. a neutral tab 841, an active tab 842 (843,844) and an earth tab 845). Tracks 10 mm wide of 4 oz copper are used onthe Mother Board 801, connecting the connection tabs (841, 842, 843,844, 845) to the other components, to enable the copper traces to carrythe expected surge currents. As with the first example, the screwlesstype connectors (101, 102, 103, 104, 121, 122, 123, 124, 125) used inthis embodiment have a tap point above the main connection opening,which are secondary connection points that permit a flat auxiliaryconductor to be inserted into the connection point above the mainconnection point and provided under the same tensioning mechanism as themain connection such that it is clamped with the same clamping force asthe main connection to provide reliable connection between the internalconductor of the connector (connecting through to the other side of theconnector), the bus bar inserted into the main connector opening and theauxiliary conductor (which is in this case a tab (841, 842, 843, 844,845) of the Mother Board 801). The voltage tap inputs of the screwlesstype connectors are not intended to carry the full load current passingthrough the connector, and are typically used to connect monitoringequipment and the like. In this case the voltage tap inputs must carryintermittent surge currents which might be many times greater than thenormal load current supplied through the SRF but will only last for andextremely short period of time.

Referring to FIGS. 7A & 7B, Capacitors C713, C714 and C715 (three eachof capacitors C713, C714, C715 will be required in the 3 phase exampleas seen in FIGS. 8 & 9), which may be typically in the range of 5-50 μf,are mounted on the Mother Board 801 and connected between the cleanactive busbar 114 (115, 116) and the clean neutral busbar 113 to furthercondition the power after the current passes through the inductors 109and 110 (111, 112).

Referring to FIG. 9, one Varistor Cartridge Tray 901 (902, 903) ismounted on the Mother Board 801 for each phase of the supply. IdenticalVaristor Cartridges 911 (912, 913) (one removed in FIG. 9) are carriedin each Varistor Cartridge tray 901 (902, 903) and each contains thesurge protection elements (MOVs) and status monitoring and indicatingcircuits, which will be described below. The Varistor Cartridge 911(912, 913) allows hot removal and replacement of the MOVs protecting theload, without disruption of the supply to the load. In the case of arailway signalling system this permits the railway to continue operationwhile replacement is being performed without danger to rail crews,passengers or maintenance staff. Connection to the circuitry within eachVaristor Cartridge 911 (912, 913) is via covered co-operating bananasockets 1021, 1022, 1023, 1024 1025, 1026, 1027 & 1028 and plugs 1011,1012, 1013, 1014 1015, 1016, 1017 & 1018 mounted respectively on theMother Board 801 and an internal PCB 1008 of the Varistor Cartridge 911(912, 913). Referring to FIG. 7B, the following connection points areprovided for each Varistor Cartridge of a 3 phase system:

1) Clean Neutral 404 (405, 406);

2) Earth 401 (402, 403);

3) Remote Monitor Signal Output 421 (422, 423);

4) Clean Active 407 (408, 409).

5) Raw Neutral 701 (702, 703);

6) Raw Active 704 (705, 706).

7) Clean Active (adjacent phase—3 phase only) 1108 (1109, 1110), (seeFIG. 11—not shown in FIGS. 7A & 7B which only shows a single phasesystem);

8) Raw Active (adjacent phase) 1111 (1112, 1113), (see FIG. 11—not shownin FIGS. 7A & 7B which only shows a single phase system);

Each of these connection points on the Mother Board are fitted with asocket for a banana plug with the mating plug connected to the circuitin the mating cartridge. Referring to FIG. 9 the banana sockets areillustrated for one phase as follows:

1) Clean Neutral 1021;

2) Earth 1022;

3) Clean Active (adjacent phase) 1023;

3) Remote Monitor Signal Output 1024;

54) Clean Active 1025.

5) Raw Neutral 1026;

7) Raw Active (adjacent phase) 1027;

6) Raw Active 1028.

Location of these connectors on the Mother Board 801 will vary slightlydepending on the specification and voltage rating of the SRF to providekeying to prevent connection of an incorrect cartridge.

As with the previous example a locking-type 4-pin connector socket 244is soldered onto a landing 424 on the Mother Board 801 to provideconnection of the Remote Monitoring Interface. As well as the 3 remotestatus monitoring signals, a clean neutral is taken out through the4-pin connector 244 to provide a signal return path. Referring to FIGS.2, 8 and 9, a 4-pin plug 243 on the free end of a 4-core cable 242connects the Remote Monitor Interface in housing 241 to the Mother Board801 via the 4-pin socket 244. The housing 241 mounted on the DIN rail231 houses the Remote Monitor Interface electronics including the SolidState Relays SSR1 & SSR2 and a triple screwless type connector providingvoltage free contacts which can be wired to provide status signals to aremote monitoring system (see FIG. 1).

Referring to FIG. 10, an exploded view of the Varistor Cartridge 911(912, 913) for a 3 phase system is illustrated. Three such cartridgesare used in a 3 phase system and the cartridge for a 3 phase system isphysically similar to a cartridge for a single phase system but has someminor variations related to keying of the cartridge with the tray 901(902, 903) and the circuit within the cartridge is modified to providephase to phase surge reduction. The Varistor Cartridge 911, 912, 913comprises a housing base 601 and housing cover 602 which includes ahandle portion 603. The base 601 and cover 602 are joined by resilientclips 605 and screws 607. The screws 607 engage through the PCB 1008within the cartridge, with threaded inserts 609 which are themselvesscrewed into the cover 602, to secure the base 601 to the cover 602 andlocate the PCB between the base 601 and the cover 602. Additional screws606 hold the PCB to the cover 602 when the base 601 is removed. MOVs andother circuit components are mounted on the Varistor Cartridge PCB 1008as shown in FIG. 10 (the equivalent componentry in FIG. 7 is similarlymounted in a cartridge for a single phase system). A LED package 1003(LED701 in FIG. 7B) is mounted in a hole 1001 in the casing 602 via acollet 1002 and connected to the Varistor Cartridge PCB 1008 via a cable(not shown). The LED package 1003 provides a visual indication of thestatus of the fuses in the cartridge (using a monitoring scheme asdescribed below).

Banana plugs extend below the Varistor Cartridge PCB 1008 to connectwith sockets 1021, 1022, 1023, 1024, 1025, 1026, 1027 & 1028 (not allrequired in the single phase case) connected to the Mother Board 801 andwhich extend through the cartridge tray 901 (902, 903). The circuitsconnected via these plugs and sockets are set out below:

Socket Plug Connection (tray) (Cartridge) 1 Clean Neutral 1021 1011 2Earth 1022 1012 3 Clean Active (adjacent phase) 1023 1013 4 RemoteMonitor Signal Output 1024 1014 5 Clean Active 1025 1015 6 Raw Neutral1026 1016 7 Raw Active (adjacent phase) 1027 1017 8 Raw Active 1028 1018

The earth plug 1012 is physically longer than the other plugs of thecartridge such that this connection makes first and breaks last.

Varistor Cartridges may be for single phase or 3 phase systems and maybe rated for 415V, 240V or 120V inputs. The locations of the 415, 240Vand 120V inputs on the Mother Board discriminate between 3 phase orsingle phase and 415, 240 or 120V Varistor Cartridges so that only acorrectly specified Varistor Cartridge can be inserted into a particularSRF.

MOV and Status Circuit

The operation of a Varistor Cartridge will now be described withreference to FIGS. 7B and 11. FIGS. 7A & 7B provide a compositeelectrical schematic drawing for a single phase unit but have beenmarked with references for equivalent points in the second and thirdphase circuits of a 3 phase system. However it will be noted withreference to FIG. 11 that although in a three phase system, there arethree cartridges, there are some differences in the Varistor Cartridgesof a 3 phase system. In particular the cartridges of a 3 phase systeminclude a raw active phase to phase MOV and a clean active phase tophase MOV (Phase N to Phase N−1 in each cartridge). Also in thecartridge of the 3 Phase system, there are no phase-to-earth MOVs(MOV701 & MOV705 in FIG. 7B).

1) The external (raw) and filtered (clean) active supplies 106 and 114(see also FIG. 8) are connected to the MOV stack through fuses FS701 andFS702. (The MOV's are arranged so that in each pair connected to onefuse there is a MOV between Active and Neutral and another betweenActive and Earth). Fuse FS701 connected to the clean active supply 114monitors MOV 701 (to earth) and MOV702 (to clean neutral) for shortcircuit. The line or external (raw) active supply 106 is connected tothe MOV stack through fuse FS702. Fuse FS702 monitors MOV704 (to rawneutral) and MOV705 (to earth) for short circuit.2) In the single phase circuit (FIG. 7B) two MOVs (MOV703 and MOV706)are connected between neutral and earth (MOV703 for the clean neutraland MOV706 for the raw neutral).3) In the 3 phase circuit of FIG. 11, MOV701 & MOV705 (of FIG.7B—actives to earth) are absent and phase to phase protection betweenthe assigned phase of the cartridge and the adjacent phase are providedby MOV1101 (clean) and MOV1104 (raw). Fuse FS701 connected to the cleanassigned active of the cartridge monitors MOV702 (to clean neutral) andMOV1101 (to clean adjacent phase) for short circuit. The external (raw)active assigned to the cartridge is connected to the MOV stack throughfuse FS702. Fuse FS702 monitors MOV704 (to raw neutral) and MOV1104 (toraw adjacent phase) for short circuit.4) In the single phase circuit (FIG. 713) two MOVs (MOV703 and MOV706)are connected respectively between the clean and raw neutrals and earth.In the 3 phase case (FIG. 11) MOV706 is omitted as the three SRFcartridges each carry a clean neutral-to-earth MOV.5) In FIG. 7B the circuit comprising capacitor C701, zener diode Z701,diode D701, resistors R701, and capacitor C705 is a voltage limitedhalf-wave rectifier and filter that supplies dc power to the B-seriesCMOS logic element packages U701, U702 and U703 (“N+11V”).6) Capacitor C701 and resistor R701 limit the maximum current throughZ701 to within its rating.7) Resistor R701 also limits the maximum forward surge current throughdiode D701 to a value within the rating of the diode.8) Diode D701 itself prevents capacitor C705 from being discharged onnegative half-cycles of the filtered mains during which the zener diode2701 becomes forward-biased, such that its cathode is pulled down toabout −700 mV with respect to Filtered Neutral 113.9) Fuse monitoring circuits are provided, one for each fuse. The circuitthat monitors fuse FS701 comprises capacitor C703, resistor R703, zenerdiode Z702, diode D702, resistor R704 and capacitor C704 which produce asignal F1 to drive Schmitt inverter U701C. When the signal F1 at theinput of Schmitt inverter U701C is in the HI state it indicates thatFuse FS701 is intact. The circuit that monitors fuse FS702 includes anoptical-coupler OC701, the photodiode of which is driven via capacitorC706 and resistor R709 from FS702. The collector of the phototransistorof the optical coupler OC701 is held high (when off) by resistor R705and the output is filtered by capacitor C707 and drives Schmitt inverterU701D. The input F2 to Schmitt inverter U701D when in the active HIstate indicate that fuse FS702 is blown (so the output F2 of U701D whenin the HI state will indicate that Fuse FS702 is intact).10) The capacitor C702 (100 nF) and resistor R702 (15 kΩ) form a voltagedivider/filter which provides a line frequency signal to the clock inputof the counter/divider circuit U701. Outputs of the counter/dividerprovide square wave outputs at 6.25 Hz and 0.78 Hz.11) The indicator itself, that is LED701, is a tri-colour device havingindividual red, blue and green segments, LED701/1, LED701/2 and LED702/3respectively. The red segment LED701/1 is driven by transistor F701through resistor R708 under the control of gate U702A, the blue segmentLED701/2 is driven by F702 through R707 under the control of gate U702Band the green segment LED701/3 is driven by F703 through R706 under thecontrol of gate U701B. The outputs of all 3 gates are active LO.12) Under normal circumstances with both fuses intact signal F1 will beHI and signal F2 will be LO. The output of gate U701B will be LO so F703will be switched on. The input combinations of neither U702A nor U702Bwill be fulfilled so both F701 and F702 will be switched off. So LED701will glow green steadily to indicate that both fuses are intact and theload connected to the SRF is fully protected. RMIN will be connected toN+11V through R710.13) If FS701 alone has operated, F1 will become LO thereby driving theoutputs of gates U701A and U701B both HI. The input combination of U702Balone will be fulfilled whenever the 0.78 Hz signal from U703 is HI soF702 will be switched on and off at 0.78 Hz. The input combinations ofneither U701B nor U702A will be fulfilled so both F701 and F703 will beswitched off. So LED701 will flash blue at 0.78 Hz to indicate that oneof the MOV banks inside that Cartridge has been overstressed, istherefore unreliable and has been disconnected (the Internal Bank inthis case) meaning that the MOV cartridge should be replaced forthwith.RMIN will be disconnected from N+11V because F703 is switched off.14) The effect of FS702 alone becoming operated is identical to thatjust described for the case of FS701 being operated.15) When both FS701 and FS702 have operated both F1 and F2 are HI. Theinput combination of U702A alone will be fulfilled whenever the 0.78 Hzand 6.25 Hz signals from U703 are both HI so F701 will be switched onand off 4 times during every HI phase of the 0.78 Hz signal with apattern termed “Burst Flashing”. The input combinations of neither U701Bnor U702B will be fulfilled so both F702 and F703 will be switched off.So LED701 will burst flash red to indicate that both MOV banks insidethat Cartridge have been overstressed, are therefore unreliable and havebeen disconnected, meaning that the MOV cartridge must be replacedurgently. RMIN will be disconnected from N+11V because F703 is switchedoff.Remote Monitor Interface16) The (optional) Remote Monitor Interface assembly is designed to bedeployed in either single- or 3-phase SRF's. Its specific purpose is toprovide a means of reporting to some off-site location the condition ofthe Varistor Cartridge and hence the level of protection of the loadconnected to the SRF.17) In the single-phase case as depicted, the 3 independent inputs tothe Remote Monitor Interface are shorted together. So when signal RMINis 1.0, NPN bipolar junction transistors Q701 and Q702 are both cut-offand the voltage at the junction of resistors R711 and R712 is zero,hence there is no path for current through LED702 or through the inputLED's of solid state relays SSR701 and SSR702, whose respective outputcontacts are in their de-energised states.18) When signal RMIN is HI, NPN transistors Q701 and Q702 are bothsaturated, and the junction of resistors R711 and R712 is connected toN+11V. Current can now flow through LED702 and the input LEDs of thesolid state relays SSR701 and SSR702 so LED702 glow, and the outputcontacts of solid state relays SSR701 and SSR702 are driven into theirrespective energised states.19) The truth table presented below summarises the behaviour of Leddevice LED701 and signal RMIN in response to all combinations of fusestates for a fully equipped dual redundancy Varistor Cartridge.

FS701 (F1) FS702 (F2) LED701 RMIN/LED702 OPERATED OPERATED Red BURST OFFOPERATED INTACT Blue 1 Hz FLASH OFF INTACT OPERATED Blue 1 Hz FLASH OFFINTACT INTACT Green ON STEADY ON STEADY

It will be appreciated by persons skilled in the art that numerousvariations and/or modifications may be made to the above-describedembodiments, without departing from the broad general scope of thepresent disclosure. The present embodiments are, therefore, to beconsidered in all respects as illustrative and not restrictive.

The invention claimed is:
 1. A surge reduction filter (SRF) comprising:a cartridge including a cartridge housing and a plurality of contactpoints; a first active connection point for connection to an active lineof an AC power supply, and a first neutral connection point forconnection to a neutral line of the AC power supply, the active andneutral connection points being located to be accessible from outsidethe cartridge; a first fuse and a first surge protection elementelectrically connected in series between the first active connectionpoint and the first neutral connection point; a status circuit tomonitor the first surge protection element; an indicator electricallyconnected to the status circuit to indicate at least a normal status anda fault status of the first surge protection element, the status circuitdetecting a change in voltage at a point between the first fuse and thefirst surge protection element and creating a fault indication when avoltage change is detected due to the first fuse operating; a cartridgetray into which the cartridge is insertable; a plurality of connectorsprojecting from a base of the cartridge tray; a plurality ofco-operating connectors projecting from the cartridge and associatedwith the plurality of contact points of the cartridge, wherein theplurality of co-operating connectors co-operate with the plurality ofconnectors of the base to connect the status circuit to an externalcircuit comprising the AC supply, and wherein each of the active lineand the neutral line of the AC power supply is a single piece of solidmetal conductor with no intermediate joins or connections and beingrouted under the tray.
 2. The SRF of claim 1 wherein one side of thefirst fuse is connected to the first active connection point and thefirst surge protection element is connected between the second side ofthe first fuse and the first neutral connection point.
 3. The SRF ofclaim 1 wherein the cartridge includes an earth connection point, asecond fuse and a second surge protection element, the earth connectionpoint being located to be accessible from outside the cartridge, and thesecond fuse and the second surge protection element are connected inseries between the first active connection point and the earthconnection point.
 4. The SRF of claim 3 wherein one side of the secondfuse is connected to the first active connection point and the secondsurge protection element is connected between the second side of thesecond fuse and the earth connection point.
 5. The SRF of claim 3wherein the cartridge includes a third surge protection elementconnected between the second side of the first fuse and the earthconnection point.
 6. The SRF of claim 3 wherein the cartridge includes afourth surge protection element connected between the second side of thesecond fuse and the first neutral connection point.
 7. The SRF of claim1 wherein the cartridge includes a second active connection point forconnection to a different phase of the AC power supply with respect tothe phase of the first active connection point, a second fuse and asecond surge protection element, the second active connection pointbeing located to be accessible from outside the cartridge, and thesecond fuse and the second surge protection element are connected inseries between the first active connection point and the second activeconnection point.
 8. The SRF of claim 7 wherein one side of the secondfuse is connected to the first active connection point and the secondsurge protection element is connected between the second side of thesecond fuse and the second active connection point.
 9. The SRF of claim7 wherein the cartridge includes a third surge protection elementconnected between the second side of the first fuse and the secondactive connection point.
 10. The SRF of claim 7 wherein the cartridgeincludes a fourth surge protection element connected between the secondside of the second fuse and the first neutral connection point.
 11. TheSRF of claim 3 wherein the status circuit monitors the state of each ofthe first and second fuses and produces a different fault indicationwhen one fuse is operated compared to a fault indication produced whentwo fuses are operated.
 12. The SRF of claim 3 wherein the cartridgeincludes a fifth surge protection element connected between the firstneutral connection point and the earth connection point.
 13. The SRF ofclaim 12 wherein the cartridge includes a sixth surge protection elementconnected between the first neutral connection point and the earthconnection point.
 14. The SRF of claim 1 wherein the cartridge includessecond active connection point, a second neutral connection point, asecond fuse and a second surge protection element, the second activeconnection point and the second neutral connection point being locatedto be accessible from outside the cartridge, and the second fuse and thesecond surge protection element are connected in series between thesecond active connection point and the second neutral connection point.15. The SRF of claim 14 wherein the cartridge includes an earthconnection point, and a third surge protection element, the earthconnection point being located to be accessible from outside thecartridge, and the first fuse and the third surge protection element areconnected in series between the first active connection point and theearth connection point.
 16. The SRF of claim 15 wherein the third surgeprotection element is connected between the second side of the firstfuse and the earth connection point.
 17. The SRF of claim 14 wherein thecartridge includes a fourth surge protection element connected betweenthe second side of the second fuse and the earth connection point. 18.The SRF of claim 1 wherein the cartridge includes a second activeconnection point for connection to a different phase of the AC powersupply with respect to the phase of the first active connection point,and a second surge protection element, the second active connectionpoint being located to be accessible from outside the cartridge, and thefirst fuse and the second surge protection element are connected inseries between the first active connection point and the second activeconnection point.
 19. The SRF of claim 18 wherein one side of the firstfuse is connected to the first active connection point and the secondsurge protection element is connected between the second side of thefirst fuse and the second active connection point.
 20. The SRF of claim18 wherein the cartridge includes a third active connection point forconnection to a third active supply line of the AC power supply, and asecond neutral connection point for connection to a second neutral lineof the AC power supply, the third active connection point and the secondneutral connection point being located to be accessible from outside thecartridge, a second fuse and a third surge protection elementelectrically connected in series between the third active connectionpoint and the second neutral connection point.
 21. The SRF of claim 20wherein the cartridge includes a fourth active connection point forconnection to a different phase of the AC power supply with respect tothe phase of the third active connection point, and a fourth surgeprotection element, the fourth active connection point being located tobe accessible from outside the cartridge, and the second fuse and thefourth surge protection element are connected in series between thefourth active connection point and an earth connection point.
 22. TheSRF of claim 21 wherein one side of the second fuse is connected to thethird active connection point and the fourth surge protection element isconnected between the second side of the second fuse and the fourthactive connection point.
 23. The SRF of claim 20 wherein the first andsecond active connection points are arranged to be connected to filteredactive supply lines of the AC supply, the first neutral connection pointis arranged to be connected to a filtered neutral supply line of the ACsupply, the third and fourth active connection points are arranged to beconnected to unfiltered active supply lines of the AC supply and thesecond neutral connection point is arranged to be connected to anunfiltered neutral line of the AC power supply.
 24. The SRF of claim 14wherein the status circuit monitors the state of each of the first andsecond fuses and produces a different fault indication when one fuse isoperated compared to a fault indication produced when two fuses areoperated.
 25. The SRF of claim 15 wherein the cartridge includes a fifthsurge protection element connected between the first neutral connectionpoint and the earth connection point.
 26. The SRF as claimed in claim 1wherein the cartridge tray and the cartridge are co-operatively keyedfor correct orientation of insertion of the cartridge into the cartridgetray.
 27. The SRF as claimed in claim 1 wherein the connector locationin the cartridge tray and cartridge varies according to a cartridgevoltage rating to co-operatively key the cartridge and cartridge tray toprevent connection of an incorrectly rated cartridge.
 28. The SRF asclaimed in claim 1 wherein the cartridge includes a status indicatingLED which indicates a status of the surge protection elements monitoredby the status circuit, the status circuit driving the LED to indicate afault condition by changing a state of emission of the LED.
 29. The SRFas claimed in claim 1 wherein the cartridge includes a monitoringconnection point and the status circuit includes a remote monitoringoutput connected to the monitoring connection point, co-operatingconnectors projecting from the base of the cartridge tray and thecartridge associated with the monitoring connection point to connect theremote monitoring output to a remote monitor interface.
 30. The SRF asclaimed in claim 1 wherein the surge protection element is an MOV.